It is well-known in the prior art to encapsulate various types of electrical devices with insulating resinous compositions. Numerous problems have been encountered in such past practice due to the severe stresses that are often applied to the insulating resins by the operating conditions of the associated apparatus. For example, when field poles of electric motors are provided with resinous insulating compositions on their coil windings, the insulating compositions are frequently subjected to extensive thermal cycling, mechanical vibration and often some abrasion, as well as being exposed to contaminants. The recognition of such problems has led to the development of various combinations of insulating materials for such field pole assemblies to assure maintenance of effective insulating characteristics for the assemblies for reasonably extended operating lifespans. One example of a recently introduced type of field pole assembly for dynamoelectric machines, which incorporates such a complex insulating systems is disclosed in co-pending U.S. patent application, Ser. No. 230,471, which was filed on Feb. 2, 1981, and is assigned to the assignee of the present invention.
In addition to the several problems encountered in manufacturing and applying such complex insulated field pole coil assemblies, as described in the foregoing patent application, a further significant disadvantage of resinous insulating systems has been discovered. It has been found that in some prior art field pole and coil assembly structures that use resinous insulating systems, voids or cracks are often formed in the resins, either when they are applied and cured, or during subsequent operation of the apparatus. Air trapped in such voids or cracks becomes ionized under the high voltage that is applied to the field coil during operation of the machine. This ionized air breaks down causing corona, which, in turn, damages the insulation further, often leading to complete failure of the coil. The desirability of forming void-free insulating systems to at least partially obviate the problem of corona formation and resultant insulation destruction has been recognized and various solutions for it have been proposed. For example, U.S. Pat. No. 3,434,087 discloses a crack-resistant casting composition for an electrical apparatus that is subject to cyclical thermal expansion and contraction. In that patent it is proposed to make an insulating casting compound having a coefficient of expansion that is very close to the thermal co-efficient of expansion of the metal apparatus on which the insulating resin is to be mounted. Thus, in operation of the system the cast insulating composition and the associated apparatus expand and contract at substantially the same rates, thereby reducing the risk of crack formation between the cast insulating composition and the associated metal apparatus.
In another U.S. Pat. No. 3,112,357, issued Nov. 26, 1963, there is disclosed a crack-free insulated conductor and method for making same. According to that invention a laminated insulation sheet is formed in a manner that avoids the development of cracks between adjacent laminations of the sheet. In a preferred embodiment a stretchable lamination, such as crepe paper, is impregnated with insulating resinous composition in an attempt to avoid cracking between the laminations of the insulating sheet.
In addition to those relatively early examples of attempts to avoid cracking of the insulation or the formation of cracks between layers of resinous insulation and adjacent electrical devices, several earlier inventors have disclosed methods for making void-free resinous insulating mica sheet materials for insulating coils of motors and generators or other electrical machines. For example, U.S. Pat. No. 4,013,987, which issued Mar. 22, 1977, discloses a micaceous material supported on a pliable fiberous sheet backing that is impregnated with an admixture of ingredients, including a viscous liquid epoxy resin and a suitable catalyst. The invention disclosed in that patent and the invention disclosed in U.S. Pat. No. 4,112,183, which issued Sept. 5, 1978, both involve the use of void-free mica insulating material mounted on flexible high-voltage insulating tape for insulating the coils of electrical machinery. The mice loaded tapes used in those inventions are applied in electrical apparatus by subsequently impregnating the tapes in a vacuum pressure impregnation process; thus, a disadvantage of such systems is the inherent relatively high expense involved in applying the tape and finally curing it to complete the insulation of an assembly.
A further problem involved in using insulating resins to hold electrical coil insulation in place and fill voids in the assembly to prevent the formation of corona is that the resins tend to drain out of parts of the insulation system before the resins can be cured. Efforts have been made to develop tapes that would enable insulating resin to flow into the insulation but would then prevent the resin from draining rapidly out of the system. In addition, various numbers of layers of binding tape or the application of multiply lapped tape to an insulated coil have been tried. For example, U.S. Pat. Nos. 3,759,734, issued Sept. 18, 1973 and 3,845,438, issued Oct. 29, 1974, disclose electrical coil insulation systems in which lap wound, open weave tapes are used to hold swellable resins in place adjacent an electric coil for a sufficient period of time to enable the resins to be cured before voids can be formed in the coil, by premature drainage of the resins. The inventions disclosed in those patents utilize open weave tapes having thermally-stable fibers with an elongation of at least two percent in the warp direction in order to allow a solventless resin to flow past the tapes to a conductor and then to substantially secure the resins from draining away from the conductor before they are cured.